1. Field Of The Invention
Oil and gas wells are typically drilled in a generally vertical orientation, that is, a borehole extends from the earth's surface generally vertically to penetrate oil and gas bearing subterranean formations. In recent years renewed interest has developed in the enhancement of oil or gas producing capabilities of a borehole by drilling the lower portion thereof generally horizontally. In some formations production can be dramatically improved by extending the well horizontally in the formation, exposing the borehole to the proximity of a greater portion of the formation than is possible with vertical drilling.
In vertical drilling the weight of the drill string, which is made up of a sequence of drill collars threadably secured to each other, augments the drilling efforts. Drilling is usually accomplished by rotating a drill string to, in turn, rotate a drill bit at the bottom of the drill string. The weight of the drill collars, particularly those immediately above the bit, is used to apply drilling pressure of the bit against the formation being drilled. Accordingly, in the usual practive of vertically drilling oil and gas wells the weight of the drill string is not normally a limiting factor and, in fact, the weight of the drill string is used advantageously. However, when a hole extends horizontally the weight of the drill collars in the horizontal portion is of no advantage. The typical horizontally bored oil or gas well extends generally vertically for a major portion of the borehole length. As the borehole approaches the producing formation, the borehole is slanted and is converted into a bend or turn in the borehole that can be as much as 90 degrees, the portion of the borehole after the turn being substantially horizontal. A problem with drilling horizontal boreholes is the fact that the drill string must be pushed horizontally as the borehole advances. Thus, increased weight of the drill collars is a disadvantage.
Another problem encountered in drilling horizontal boreholes is that rotary drill strings cannot be effectively employed. For this reason, the typical means of drilling horizontally in a borehole is to use what is known as "turbine drill"--that is, a drilling system employing a motor producing rotational torque in response to fluid flow therethrough, the rotational torque being applied to a drill bit. Turbine drilling requires a fairly high flow rate of drilling fluid. The drilling fluid serves three main purposes. First, the flow of the drilling fluid through the interior of the drill string and out through the turbine provides the rotational torque for rotating the drill bit. Second, the circulating drilling fluid serves to carry away cuttings produced by the drill bit that must be carried back to the earth's surface to leave the drilled hole open. Third, the circulating drilling fluid serves to cool the bit as it works against the formation being drilled.
A serious problem encountered with the use of the typical cylindrical drill strings made up of a sequence of lengths of cylindrical drill collars is that of preventing the drill collar from becoming stuck in the horizontal portion of the borehole. As a cylindrical drill collar lies horizontally in a borehole in a non-rotating position the cuttings produced by the turbine rotating drill bit tend to settle and collect around the cylindrical drill collar in the area between the drill collar and the interior of the horizontal borehole. If sufficient cuttings settle and compact in this manner the drill string can be stuck so that it cannot be pulled out of or retrieved from the borehole.
In object of this invention is to provide an improved drill collar for use in drilling a horizontal borehole and to eliminate the cronic problem of rotating the drilling string from surface to bit.
2. Related Background Art
Others have given consideration to the desirability of improved drill collars. U.S. Pat. No. 3,525,237 to Lari entitled "Drill-Stem", issued Aug. 25, 1970, shows an integral drill collar with a generally rectangular configuration. The drill collar is of a unitary construction, that is, the threaded end portions and the intermediate portion are of the same material and the exterior configuration is somewhat triangular but more in the shape of a curved equilateral triangle, with equal axial moments of inertia as to every axis that passes through the center of the base circle of the cross-section of the triangle. Lari provides a drill collar that is non-circular in cross-section but wherein the cross-section does not have the advantages of a distinct triangular shape so as to allow, as the drill collar lies in the horizontal borehole, free fluid flow passages between the exterior of the drill collar and the cylindrical borehole. Further, Lari provides a drill collar of a unified construction so that all portions thereof must be of a material having sufficient strength to accept threads so that drill collars can be joined together.
U.S. Pat. No. 3,237,427 to Scarborough entitled "Drill Collar", issued Mar. 1, 1966, shows a drill collar in which the cross-section thereof is rectangular. The rectangular configuration is an improvement over a cylindrical drill collar if used for horizontal drilling but still does not supply adequate areas of fluid circulation in the annular space between the exterior of the drill collar and the interior cylindrical borehole. Further, Scarborough, like Lair, is of a unified construction and thus not susceptible of being formed of lightweight material.
Other patents that show drill collars of various constructions are the following:
______________________________________ 2,263,579 Hokanson Sectional Drill Rod Nov 25, 1941 2,330,564 Dyer Mud Ejection Control Sep 28, 1943 3,067,593 McCool Integral Tool Joint Drill Dec 11, 1962 Pipe 3,175,374 Toelke Tubular Member For Mar 30, 1965 Use In Well Drilling Operations 3,237,427 Scarborough Drill Collar Mar 01, 1963 3,338,069 Ortloff Rotary Drill Collar Aug 29, 1967 3,525,237 Lari Drill-Stem Aug 25, 1970 4,378,057 O'Connell Coupling Structure For Mar 29, 1983 A Compound Drill Stem ______________________________________
These patents are representative of the state of the art of drill collars as used in drilling oil or gas wells.